Multi-speed manual gearboxes of motor vehicles commonly have a countershaft design and comprise at least two axially parallel gearbox shafts, which can be brought into a driving connection with each other selectively by means of several gear steps having different gear ratios. For a manual gearbox designed to be installed longitudinally in a motor vehicle, the gearbox shafts in question are usually the countershaft, which has a driving connection to the input shaft via an input constant, and the output shaft. For a manual gearbox designed to be installed transversely in a motor vehicle, the gearbox shafts in question are usually formed by the input shaft and the output shaft. The gear steps are mostly designed as spur gear pairs that each comprise a fixed gear disposed on the one gearbox shaft in a rotationally fixed manner and a freewheeling gear rotatably supported on the other gearbox shaft.
To engage a gear step, i.e., to establish a driving connection between the two gearbox shafts having the gear ratio of the gear step in question, a gear coupling is associated with each freewheeling gear, by means of which gear coupling the freewheeling gear can be connected to the gearbox shaft in question in a rotationally fixed manner. For a manual gearbox designed for longitudinal installation, the output shaft is generally disposed coaxially adjacent to the input shaft and can be connected to the input shaft by means of a gear coupling, whereby an additional direct gear step is available. The freewheeling gears of axially adjacent spur gear pairs are preferably disposed on the same gearbox shaft at least in pairs so that the gear couplings can be combined in pairs into shifting groups each having a common selector sleeve.
For the transmission of selecting and shifting movements within the gearbox, a selector shaft disposed axially parallel to the gearbox shafts can be provided, as assumed here. The selector shaft is axially movable, is supported in the gearbox housing so as to be rotatable about the longitudinal axis of the selector shaft, and can be selectively coupled to several shift forks each guided on a shift rail and/or to several shift rockers by means of an associated selector finger. The shift forks and the shift rockers each engage in a form-locking manner with the selector sleeve of an associated shifting group and can axially move the selector sleeve in order to engage and disengage gear steps. The selector fingers are disposed on the selector shaft with axial offset and generally also with circumferential offset, and therefore only one of the selector fingers engages with a carrier of a shift rail or of a shift rocker when the selector shaft rotates, which corresponds to a selection procedure or the selection of a shift gate. As the result of a subsequent axial movement of the selector shaft, the previously coupled shift fork or shift rocker is axially moved or is pivoted about its rotational axis, whereby the associated selector sleeve is axially moved, which corresponds to a shifting procedure or the engagement and disengagement of one of the gears of the selected shift gate.
The shift rails or shift forks and the shift rockers are usually secured in a friction-locking manner by means of a spring locking mechanism in the positions corresponding to the neutral position and the shift positions of the associated selector sleeve. However, under unfavorable operating conditions, such as during driving on a poor road surface, during which sever shaking can occur, and during heavy braking maneuvers or fast cornering that causes high inertial forces, this can be insufficient to reliably prevent the simultaneous deflection of at least two shift elements (shift forks and/or shift rockers) from their neutral position. However, the basic prerequisite for the safe operation of the manual gearbox and in particular for the trouble-free performance of selection procedures and shifting procedures is that the shift elements that are not coupled to the shifting device remain in their neutral position. Therefore, many styles of shift locking devices or shift securing devices for multi-speed manual gearboxes have been proposed, by means of which the shift elements that are not coupled can be locked in their neutral position in a form-locking manner independently, i.e., without the aid of locking elements operated by means of external energy.
Shift securing devices for manual gearboxes having shift forks guided on shift rails are known from DE 21 32 736 A, DE 37 30 230 C1, and DE 196 53 172 A1, wherein during the axial movement of one of the shift rails due to shifting, one or more locking elements that can move transversely to the shift rails are pressed into a respective locking groove of the other shift rails by a control bevel of the axially moved shift rail and these other shift rails are thus locked in their neutral position. In the shift securing device according to DE 21 32 736 A, the locking elements are designed as locking pins or locking balls disposed in transverse holes in the housing. The shift securing device according to DE 37 30 230 C1 comprises a single locking element designed as a locking plate, which is disposed perpendicular to the shift rails and engages by means of the recesses associated with edges in a respective annular groove of the shift rails not coupled. In the shift securing device according to DE 196 53 172 A1, the locking elements are designed as blocking panels that can be rotated about an axially parallel axis. These known shift securing devices require little packaging space, but they require high precision in the manufacture of the components in question and are difficult to assemble.
In DE 40 17 957 A1, a shift securing device for manual gearboxes having shift rockers is described, wherein the locking elements, which each act between two shift rockers, are designed as rocker arms supported on the housing. When one of the shift rockers rotates as a result of a shift, the other shift rocker is locked in its neutral position by the engagement of a locking cam of the rocker arm, which is partially pivoted along, in a locking groove. In order to couple two axially adjacent rocker arms, an arrangement of the locking cams and the locking grooves in the opposite direction on the middle shift rocker and a push rod that connects the two rocker arms in an articulated manner are provided. However, this known shift securing device requires a large installation space and is relatively complex to assemble.
Because manual gearboxes can comprise both shift forks guided on shift rails as well as shift rockers as shift elements, in particular in order to achieve compact dimensions, special shift securing devices in which the two styles of shift elements are combined are required for this purpose. Thus in DE 41 18 931 A1 a shift securing device for a manual gearbox having two shift forks guided on shift rails and two shift rockers is described, wherein a locking element of the shift rails designed as a ball or as a blocking plate and a locking element of the shift rockers designed as a rocker arm are operatively connected to each other by means of a push lever connected to the rocker arm in an articulated manner and a swiveling lever supported on the housing.
All previously mentioned shift securing devices have the functional disadvantage that the shift elements (shift forks and/or shift rockers) that are not coupled are not secured in a form-locking manner until the coupled shift element moves or pivots from its neutral position.
By contrast, for manual gearboxes having shift forks guided on the selector shaft in an axially movable manner, shift securing devices are known for which the shift forks that are not coupled are already secured in a form-locking manner at the time of the selection procedure, i.e., the coupling of one of the shift forks by rotation of the selector shaft. Thus a shift securing apparatus having a locking body that is locked to the housing in the axial direction and that can rotated with the selector shaft is known from DE 30 03 076 C2, the locking body being designed as a longitudinally slotted sleeve and being disposed on the selector shaft by means of the single selector finger. The single locking element is designed as a radially oriented blocking plate that is radially slotted in the region of the selector finger, that engages in a respective opening disposed on each shifting arm of the shift forks, and that is axially fixed relative to the gearbox housing by means of a slotted screw. Because of the longitudinal slot of the sleeve and the radial slot of the blocking plate, only the shift forks coupled by the engagement of the selector finger in the opening of the shifting arm in question can be axially moved with the selector shaft.
In a functionally similar shift securing device known from DE 199 01 055 A1, the locking body is designed as an elongated carrier plate guided radially between the selector shaft and the shift forks in longitudinal grooves of the shift forks, the carrier plate being axially fixed by means of a circumferential slot, in which a bolt fastened to the housing engages. The shift forks that are not coupled are locked in a form-closed manner by the engagement of locking pins disposed on the carrier plate in inner radial grooves of the shift forks, whereas the locking pins associated with the coupled shift fork lie in the region of axial openings of the radial groove in question. However, this style of manual gearbox having shift forks guided on the selector shaft in an axially movable manner is relatively rare and is not practical for achieving compact dimensions for manual gearboxes having more than three shift elements or shift gates.